Process for improving stress-corrosion resistance of age-hardenable alloys



May 19, 19

Filed May 11,

OF AGE-HARDENABLE ALLOYS HDNI I-IUVflOS 33d SNOl NI 98381.9 EllVWll'mHQVUBAV G. THOMA 9 PROCESS FOR IMPROVING STRESS-CORROSION RESISTANCE 2Sheets-Sheet l iT 18 i9 20 21 1 2 1 3 1 4 1 5 AVERAGE TOTAL AGING TIMEIN HOURS INVENTOR. GARETH THOMAS May 19, 1964 OM s 33,839

G. TH A PROCESS FOR IMPROVING STRESS-CORROSION RESISTAECE OFAGE-HARDENABLE ALLOYS Filed May 11, 1961 2 Sheets-Sheet 2 Specimens Agedfor 24 Hour Still Uncracked After 6 Moni'hs Specimens Aged for 10 8. 24Hours Still Uncracked After 6 Months 8 9 1O 11 42 AVERAGE TOTAL AGINGTIME IN HOURS s 2 8 a 2 sxvo NI am nolsouaooseams aovamw INVENTOR.

N GARETH THOMAS A 7' TORNE Y United States Patent 3 133 839 PROCESS FORrMPRovnsG STRESS-CORROSION RESISTANCE 0F AGE-HARDENABLE ALLOYS GarethThomas, 1357 Glendale Ave., Berkeley 8, Calif. Filed May 11, 1961, Ser.No. 109,429

5 Claims. (Cl. 148-12.7)

The present invention relates to a process for improving thestress-corrosion resistance of age-hardenable alloys and, moreparticularly, to a process for improving the stress-corrosion resistanceof age-hardenable alloys based on the aluminum-zinc-magnesium system.Stress-corrosion concerns the behavior of metals under the combinedaction of corrosive environment and high tensile stresses, and refers toboth the acceleration of corrosion by stress and stress-corrosioncracking.

As used herein, the term alloys based on the aluminum-Zinc-magnesiumsystem refers to alloys consisting essentially of zinc in an amountbetween about two and about ten percent by weight, magnesium in anamount between about one and about four percent by weight, and thebalance aluminum with or without minor additions of copper (less thanone percent by weight), manganese (less than 0.5 percent by weight),chromium (less than 0.2 percent by Weight, etc. and/ or impurities suchas iron (less than 0.3 percent by Weight), silicon (less than 0.2percent by weight), titanium (less than 0.1 percent by weight), etc.

Heretofore, it has been shown that the mechanical strength of the lightalloys based on the aluminum-zincmagnesium system can be increased bysubjecting the alloys to solution heat treatment and subsequent aging.Solution heat treatment is a process wherein an alloy is heatedsufficiently to etfect a solution of the soluble alloying elements andthen cooled rapidly to hold the elements in solution. When solution heattreated alloys are allowed to stand or age, their strength and hardnessusually continue to increase beyond the values existing immediatelyafter the solution heat treatment. If this gradual change in thephysical properties of the alloy takes place at room temperature, it iscalled natural or spontaneous aging; if it takes place at temperaturesabove room temperature, 'it is termed artificial aging or precipitationheat treatment.

Although solution heat treated and aged alloys based on thealuminum-zinc-magnesium system have the highest available mechanicalstrength of all the age-hardenable aluminum-base alloys, they areextremely susceptible to stress-corrosion, i.e., they have a lowstress-corrosion resistance. Various alloying elements, such as copper,manganese, and chromium, have been added to these ternary alloys in anattempt to improve their resistance to stresscorrosion, but theresulting alloys are still not acceptable for many applications where arelatively high-stress-corrm sion resistance is required.

It is, therefore, the main object of the present invention to provide aprocess for improving the stress-corrosion resistance of age-hardenablealloys based on the aluminum-zinc-magnesium system.

It is another object of the inventiontto provide such a process whichwill not detract from the mechanical strength of the alloys. 1

It is a further object of the invention to provide such a processwherein precipitate-free regions next to grain boundaries are eliminatedor modified in order to prevent intercrystalline embrittlement andpreferential chemical attack.

Other aims and advantages of the invention will be apparent from thefollowing description and appended claims.

In the drawings:

FIG. 1 is a graph showing the average ultimate tensile strength (as afunction of aging time) of samples of an aluminum-zinc-magnesiurn alloytreated by the inventive process and samples of the same alloy treatedby a conventional aging process; and

FIG. 2 is a graph showing the average stress-corrosion life (as afunction of aging time) of the alloy samples of FIG. 1.

In accordance with the present invention, there is provided a processfor improving the stress-corrosion resistance of an age-hardenable alloybased on the aluminumzinc-magnesiurn system comprising partially agingsaid al-' loy, cold Working said alloy more than five per cent, andcompleting the aging of said alloy.

The problem of stress-corrosion cracking generally arises in alloyswhich have been solution heat treated and then aged to produce maximumtensile strength, where there is an absence of precipitates in theregions next to the grain boundaries. The inventive process eliminatesor modifies these precipitate-free regions, thereby preventingintercrystalline embrittlement and improving stress-corrosionresistance.

The alloy to be treated by the inventive process is first subjected to aconventional solution heat treatment. In general, the heat treatingtemperature is made as high as possible without melting any of theeutectic material present in the alloy. The preferred temperature rangefor the aluminum-zinc-magnesium alloys to be treated by the presentprocess is about 465 C. (:5" C.). The alloy is held at the hightemperature long enough for solution and diffusion to take place andproduce, as nearly as practicable, a homogeneous solid solution. Thealloy is then rapidly cooled or quenched so that there is not time forthe hardening elements to precipitate from solid solution during thecooling period, in accordance with their lower solubility at lowertemperatures. With the ternary alloys to be treated by the presentprocess, it is preferable to quench into hot water (about C.). Thesolution heat treatment increases all the mechanical properties of thealloy.

The solution heat treatment may be carried out in a molten salt bath,such as fused sodium nitrate, or in an air furnace. The furnace may beheated electrically or by gas, oil, or radiant tubes. For best results,artificial circulation should be provided. Furnaces in which products ofcombustion are circulated can be used satisfactorily if the atmospherecontains a sufficiently high percentage of car- .bon dioxide and is freefrom sulfur compounds and excessive moisture.

Following solution heat treatment, the dissolved alloying constituentstend to precipitate from solid solution in accordance with their truesolubility. This precipitation or aging is accelerated by raising thetemperature of the alloy; the preferred temperature range for theternary alloys to be treated by the present process is from about toabout C. The alloy is maintained at the increased temperature only longenough to initiate precipitation of the dissolved alloying constituents;for example, if 150 C. is chosen as the aging temperature, the partialaging time should be less than three hours. The actual time required forthis preliminary aging step depends on the particular alloy beingtreated and the particular temperature employed, but should always beless than the time required to produce maximum age hardening.

Both the preliminary and final aging steps can be carried out in afurnace or oven heated electrically or by gas. Provision should be madefor circulating the air in order to obtain a uniform temperaturethroughout the furnace, as well as more rapid transfer of heat.

After the alloy has been partially aged, it is cooled to roomtemperature and cold worked. The inventive process introduces thisintermediate deforming step into the normal aging sequence in order toallow precipitation to occur near grain boundaries during the finalaging step. The cold working introduces preferential plastic flow nearthe grain boundaries, thus introducing excess vacancies as a result ofdislocation interactions and enabling the subsequent heat treatment toeffect precipitation in the previously precipitate-free regions adjacentto the grain boundaries. The alloy must be cold worked more than aboutfive percent, but preferably less than about ten percent. Deformationsabove about ten percent create a condition in the alloy which tends toreduce the beneficial effect of the preliminary aging. The amount ofcold working is measured by the reduction in cross-sectional area of theparticular article being deformed.

Following the cold working, the alloy is again heated to a temperaturehigh enough to permit development of the desired properties in areasonable time, and the aging process is completed. The temperatureemployed in this final aging step may not be higher than that employedin the preliminary aging step. As in the first aging step, the preferredfinal aging temperature range for the subject aluminum-zinc-magnesiumalloys is from 100 to 160 C. The period required to complete the agingprocess depends on the particular alloy being treated, the particulartemperature employed, and the amount of aging affected by thepreliminary aging step. Normally, the aging process is considered to becomplete at the point of maximum strength and hardness.

In an example of the inventive process, an alloy (in sheet form)containing 5.93% zinc, 2.9% magnesium, 0.5% copper, 0.17% chromium,0.48% elemental impurities, and the balance aluminum (all percentages byweight) was solution heat treated by quenching into water at 80 C. aftera 12-hour anneal at 465 C. The solution heat treated alloy was splitinto three batches of specimens for further processing. The specimens inbatch A were subjected to a direct heat treatment at 150 C. for variousperiods up to 24 hours, with no intermediate cold working. The specimensin batch B and batch C were aged at 150 C. for 30 minutes; cooled toroom temperature; deformed five and ten percent, respectively, byrolling; and then further aged at 150 C. for various periods up to 23.5hours. Each specimen was tested for ultimate tensile strength andstress-corrosion resistance. The stress-corrosion test was carried outby bending the specimen to 90% proof stress and exposing it toindustrial atmosphere. The results of the tensile strength tests areshown in FIG. 1, while the results of the stress-corrosion resistancetests are shown in FIG. 2, the results of each test being plotted as afunction of aging time.

Referring to FIGS. 1 and 2, it can be seen that the maximum ultimatetensile strength for the batch A specimens (no intermediate coldworking) was about 35 tons per square inch, but the maximum resistanceto stresscorrosion at that strength was less than 40 days. Also, themaximum average stress-corrosion resistance out of all the batch Aspecimens was only 48 days, which is not acceptable for certainpractical applications. In the batch B specimens (5% intermediate coldworking), the stresscorrosion resistance in the specimens having themaximum ultimate tensile strength was only about 25 days, but thespecimens having an ultimate tensile strength of about 32 tons persquare inch were still uncracked after six months in thestress-corrosion test. An ultimate tensile strength of 30 tons persquare inch is adequate for many structural purposes. In batch C, thespecimens having an ultimate tensile strength of about 35 tons persquare inch were still uncracked after six months in thestress-corrosion test.

The curves in FIG. 1 indicate that the ultimate tensile strength of thealloy decreased as the amount of cold working and the aging timeincreased, whereas the curves in FIG. 2 indicate that thestress-corrosion life increased as the amount of cold working and theaging time increased. However, the rate of increase in thestresscorrosion life was much greater than the rate of decrease in theultimate tensile strength. Thus, the inventive process can increase thestress-corrosion life considerably while sacrificing only a relativelysmall amount of tensile strength; by varying the total aging time andthe amount of intermediate cold working in accordance with the curvesillustrated in FIGS. 1 and 2, the stress-corrosion resistance of thealloy can be increased to many times that of conventionally aged alloyswithout substantially detracting from the mechanical strength of thealloy. Alloys treated by the present process are especially suitable foraircraft and rocket structures.

While various specific forms and preferred ranges for the presentinvention have been herein set forth, it is not intended to limit thisinvention to the embodiments herein described, but only as set forth inthe appended claims.

What is claimed is:

1. A process for improving the stress-corrosion resistance of anage-hardenable alloy consisting essentially of zinc in an amount betweenabout one and about ten percent by weight, magnesium in an amountbetween about one and about four percent by weight, and the balancealuminum, comprising partially aging said alloy at a temperature betweenabout 100 and about 160 C., cold working said alloy between about 5 andabout 10 percent, and completing the aging of said alloy at atemperature between about 100 C. and the temperature employed in saidpartial aging step.

2. A process for improving the stress-corrosion resistance of anage-hardenable alloy consisting essentially of zinc in an amount betweenabout one and about ten percent by weight, magnesium in an amountbetween about one and about four percent by weight, and the balancealuminum, comprising aging said alloy at a temperature between about 100and about 160 C. for 30 to minutes, cold working said alloy betweenabout 5 and about 10 percent, and completing the aging of said alloy ata temperature between about C. and the temperature employed in the firstaging step.

3. A process for improving the stress-corrosion resistance of anage-hardenable alloy consisting essentially of zinc in an amount betweenabout one and about ten percent by weight, magnesium in an amountbetween about one and about four percent by weight, and the balancealuminum, comprising partially aging said alloy at a temperature betweenabout 100 and about C., cooling the partially aged alloy to roomtemperature, cold working said alloy between five and ten percent, andcompleting the aging of said alloy at a temperature between about 100 C.and the temperature employed in said partial aging step.

4. A process for improving the stress-corrosion resistance of anage-hardenable alloy consisting essentially of zinc in an amount betweenabout one and about ten percent by weight, magnesium in an amountbetween about one and about four percent by Weight, and the balancealuminum, comprising partially aging said alloy at a temperature betweenabout 100 and about 160 C., cold working said alloy between about 5 andabout 10 percent, and completing the aging of said alloy, the totalaging time and the amount of cold working efiected being adjusted toprovide the desired stress-corrosion resistance and tensile strength insaid alloy at a temperature between about 100 C. and the temperatureemployed in said partial aging step.

5. A process for improving the stress-corrosion resistance of anage-hardenable alloy consisting essentially of zinc in an amount betweenabout one and about ten percent by weight, magnesium in an amountbetween about one and about four percent by weight, and the balancealuminum, comprising aging said alloy at a temperature between about 100and 160 C. for 30 to 90 minutes, cold working said alloy between fiveand ten percent, and further aging said alloy at a temperature betweenabout 100" C. and the temperature employed in the first aging step untilthe desired stress-corrosion resistance and tensile strength areobtained in said alloy.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR IMPROVING THE STRESS-CORROSION RESISTANCE OF ANAGE-HARDENABLE ALLOY CONSISTING ESSENTIALLY OF ZINC IN AN AMOUNT BETWEENABOUT ONE AND ABOUT TEN PERCENT BY WEIGHT, MAGNESIUM IN AN AMOUNTBETWEEN ABOUT ONE AND ABOUT FOUR PERCENT BY WEIGHT, AND THE BALANCEALUMINUM, COMPRISING PARTIALLY AGING SAID ALLOY AT A TEMPERATURE BETWEENABOUT 100* AND ABOUT 160*C., COLD WORKING SAID AOOLY BETWEEN ABOUT 5 ANDABOUT 10 PERCENT, AND COMPLETING THE AGING OF SAID ALLOY AT ATEMPERATURE BETWEEN ABOUT 100*C. AND THE TEMPERATURE EMPLOYED IN SAIDPARTIAL AGING STEP.